The present invention relates generally to coherent radiation sources and specifically to the tuning of free electron lasers to different wavelengths.
Free electron lasers generate very high power pulses of coherent electromagnetic radiation by projecting a relativistic electron beam along the longitudinal axis of a drift tube where the beam interracts with a transverse, periodic magnetic field to amplify a superimposed optical wave.
The task of providing a free electron laser (FEL) which is tunable to different wavelengths is alleviated, to some degree, by the following U.S. patents, which are incorporated by reference:
U.S. Pat. No. 3,822,410 issued to Madey on July 2, 1974; PA1 U.S. Pat. No. 4,287,488 issued to Brau et al on Sept. 1, 1981; PA1 U.S. Pat. No. 4,331,936 issued to Schlesinger et al on May 25, 1982; PA1 U.S. Pat. No. 4,345,329 issued to Doucet et al on Aug. 17, 1982; and PA1 U.S. Pat. No. 4,425,649 issued to Elias et al on Jan. 10, 1984.
One example of a free electron laser is described in U.S. Pat. No. 3,822,410. This type of free electron laser operates on the principle of magnetic bremsstahlung wherein a periodic magnetic field is utilized to produce radiation. Other types of free electron lasers, more commonly referred to as the Smith-Purcell and Cerenkov lasers are described in a publication by Gover et al entitled "Operation Regimes of Cerenkov-Smith-Purcell Free Electron Lasers and R.W. Amplifiers," Optics Communications, Vol. 26, No. 3, September 1978, pp. 375-380. In these devices a slow electromagnetic wave structure and periodic waveguide is used to facilitate the interaction of the electron beam and the electromagnetic wave.
However, even with these apparent different physical principles, both the magnetic bremsstrahlung and the Cerenkov-Smith-Purcell laser have similar gain expressions, similar wave dispersion equations and similar operation regimes. The main difference between the magnetic bremsstrahlung and the Cerenkov-Smith-Purcell lasers is that the magnetic bremsstrahlung laser involves transverse modulation of the electron beam by the transverse periodic force, while the Cerenkov-Smith-Purcell lasers involve direct longitudinal modulation of the electron beam by the longitudinal component of the electric field of the electromagnetic wave. Nevertheless, the interaction between the electron beam and the electromagnetic wave is carried out through longitudinal modulation of the electron beam which is created by the ponderomotive force-effect.
Since this is a second or third order effect in the fields, the interaction between the electromagnetic wave and the electron beam in the magnetic bremsstrahlung free electron laser is much weaker than the interaction in the Smith-Purcell-Cerenkov lasers (which are first order effects). Therefore, in principle, one of the differences between the magnetic bremsstrahlung free electron laser and the Smith-Purcell-Cerenkov lasers is that the latter devices can provide higher gain.
Another difference between magnetic bremsstrahlung laser and Smith-Purcell-Cerenkov lasers is in the interaction region width, which affects the power and efficiency of the device. In this aspect the difference is in favor of the magnetic bremsstrahlung free electron laser.
Unfortunately, both types of free electron lasers, as described hereinabove, contain drawbacks which affect both the efficiency and the overall reliability of their operation. In addition to the drawbacks presented above, the above-mentioned lasers also tend to be large and bulky as well as being difficult to tune. Consequently, there remains a void in the area of free-electron lasers which needs to be filled.